Friction wheel measuring device

ABSTRACT

A distance measuring device having a variable radius frictionally driven measuring wheel is described. The repeatability of measurements made by the measuring device is improved by providing a plurality of randomly spaced apart ridges on the periphery of the measuring wheel with the long axis of the ridges extending transverse to the circumference of the wheel. Accuracy of measurement is assured by an adjusting method wherein the wheel is disengaged from the measurement surface during adjustment and the wheel moved to prevent it from following in a track previously made in the measurement surface.

United States Patent 1191 a Y 1 1 3,771,228

Culver I Nov. 13, 1973 [54] FRICTION WHEEL MEASURING DEVICE I 17,8400/1888 Great Britain 33/141 C [75] Inventor: lrven H. Culver, Playa DelRey,

Cahf' Primary Examiner-Harry N. l-laroian [73] Assignee: Primus Mfg.,Inc., San Lorenzo, Attorney-Richard D. Seibel et a1.

[22] Filed: Oct. 12, 1971 [21] Appl. No.: 188,314 I ABSTRACT A distancemeasuring device having a variable radius 33/141 gfl frictionally drivenmeasuring wheel is described. The [58] g l /129 l30 repeatability ofmeasurements made by the measuring device is improved by providing aplurality of randomly spaced apart ridges on the periphery of themeasuring wheel with the long axis of the ridges extending transverse tothe circumference of the wheel.

[56] References Cited A ccuracy of measurement 1s assured by an ad ustmgUNITED STATES PATENTS method wherein the wheel is disengaged from the3,307,265 3/1967 Jenks et al. 33/125 M measurement surface duringadjustment and the wheel 2,110,757 3/1938 Clarke moved to prevent itfrom following in a track previ- 819,096 5/1906 Teate 33/132 ously madei the measurement f FOREIGN PATENTS OR APPLICATIONS 449,028 12/1912France 33/141 R Q 12 Claims,6 Drawing Figures H FRICTION WHEEL MEASURINGDEVICE BACKGROUND This invention relates to friction wheel distancemeasuring devices of the type described in U.S. Pat. Nos. 3,31 1,985 and3,378,929. As taught in these patents a measuring device may be mountedon a lathe carriage, for example, to traverse along the lathe bed duringa machining operation. A friction wheel on the measuring device engagesa portion of the lathe bed so as to roll along it as the carriagetraverses. In a typical embodiment a precision gear train in themeasuring device magnifies the rotation of the measuring wheel forindication on a dial indicator or the like. Direct reading or otheramplification techniques can be used as desired. Since the circumferenceof the wheel is known with precision, exact measurement of distance oftravel of the lathe carriage can be obtained by reading the indicator.Such a distance measuring instrument can be mounted on other machinetools or employed in other applications.

Precision friction wheel distance measuring devices have found wideacceptance throughout industry in many applications. The measuringdevice which is most widely used is marketed by the assignee of thisinvention in conjunction with the trademark TRAV-A- DIAL. Such measuringinstruments are calibrated in units as small as five ten-thousandths(0.0005) of an inch or 0.01 centimeter or even 0.0001 inch, and thedistance measured equals the circumference of the measuring wheel, whichis very accurately controlled, before recycling of the distanceindicator occurs. A friction wheel measuring device having a measuringcapacity many times the circumference of the measuring wheel is marketedby the assignee of this invention in conjunction with'the trademarkTEDD. Such a device may operate over distances of 100 inches or morebefore recycling of the measuring indication occurs. It was with theadvent of such extended readout measuring instruments that problems ofrepeatability were principally noticed.

Accuracy of a measuring instrument is defined as the ability of theinstrument to indicate the exact distance measured after traversing apath to be measured. Thus, for example, accuracy may be ascertained bytraversing the measuring instrument over a path known with greatprecision, such as for example as may be determined with conventionalgauge blocks or the like. If one traverses a distance measuringinstrument over a ten inch path as determined by gauge blocks, themeasuring indication should be precisely ten inches. Accuracy is ameasure of the ability of the instrument to achieve such a result.

Repeatability is a measure of the ability of the measuring device toread zero when returned to its original position after motion away fromand back to the original position. Thus, for example, a measuring devicemay be set at zero, traversed several times back and forth over a pathof 50 inches and returned to its original position. Any deviation from azero reading upon return to the original position is considered arepeatability error. It has been found that such repeatability errorsarise from mechanical hysteresis in the devices being measured or in themounting structure for the measuring device. This mechanical hysteresistypically arises from nonreciprocal elastic deformations and afterseveral such traversal cycles a substantial error may be accumulated.

A distance measuring instrument may have a high degree of accuracy andpresent no repeatability problems when operated over short distances,but the same instrument used in exactly the same mounting on the samemachine tool may show repeatability errors without any change inaccuracy when operated a number of times over great distances. Themagnitude of the repeatability error may vary for the same measuringinstrument from one machine tool to another even when the machine toolsare nominally identical.

U.S. Pat. Nos. 3,307,265 and 3,561,121 describe a friction wheeldistance measuring device wherein the friction wheel has a perimeter inthe form of an arcuate surface so that the effective radius and, hence,circumference of the measuring wheel is variable by tilting the distancemeasuring device in a direction transverse to the path along whichdistance is to be measured. Such an arrangement improves the accuracy ofdistance measurement with the device.

U.S. Pat. No..3,56l,l20 describes a technique for increasing therepeatability of measurementsmade with a friction wheel measuring deviceby skewing the device relative to the direction of the path to bemeasured.

Prior Art Adjustment Techniques FIGS. A, B, and C illustrate in plan,end, and elevation view, respectively, a distance measuring instrumentof the friction wheel variety. Although these figures represent theprior art instrument, they'also illustrate schematically an instrumentsuitable for practice of this invention.

As illustrated in these drawings there is an instrument housing 10within which a measuring wheel 11 is rotatably mounted. The measuringwheel 11 has a small portion ,of its periphery extending through an endof the housing 10. A knob 12 is coupled to the wheel 11 and hascalibration marks (not shown) readable on the top face of the measuringinstrument. Typically one full revolution of the knob corresponds to onefull revolution of the wheel. In some embodiments an arrange ment may beused wherein the knob rotates a full revolution only after severalrotations of the measuring wheel. Within the housing 10 and notillustrated herein is a conventional anti-backlash motion amplificationgear train interconnecting the wheel 11 and a dial indicator 13calibrated in thousandths of an inch or hundredths of a centimeter asmay be desired. Thus, the calibration markings on the knob 12 provide acoarse indication of the distance traversed by the wheel and the dialindicator provides a fine indication of the same. If the wheel has a 6inch circumference, the measuring indication repeats after six inches.In other embodiments not shown, an electronic sensor of wheel rotationis used which accumulates the rotations so that many circumferences canbe traversed before the indication repeats. Other indicators, motionamplifiers and the like can be used.

The measuring instrument is mounted on a movable portion 14 of a machinetool by a support base 16, shown only schematically herein. The movableportion 14 may, for example, be the carriage of a lathe or other movableportion of a machine tool. The measuring wheel 11 of the instrument isin frictional engagement with a fixed portion 17 of the machine such asa guide way of the lathe bed; this frictional engagement is assured by abias force applied to housing by the support base. Thus, as the carriagel4 traverses along the lathe bed, the wheel 11 rolls along the guide way17, thereby rotating the knob 12 and dial gauge 13. By reading the knoband dial gauge, the distance traversed can be measured.

Support base or mounting base as used herein refers to the variousmechanisms used for attaching the measuring instrument to a machine tooland providing the needed adjustments. The support base takes a varietyof forms depending on the type and make of machine tool and a number ofconventional and special purpose bases are known. The support base mayinclude a mounting base as described in U.S. Pat. No. 3,378,929 whichprovides a spring bias urging the instrument against a measurementsurface. It may also include a mounting bracket such as described inU.S. Pat. No. 3,307,265 for interconnecting the measuring instrument andthe tool and also for adjusting the angular relations therebetween. Avariety of other means for adjusting position, angle and bias force ofthe instrument will be apparent to one skilled in the art, and thesupport or mounting base is, therefore, shown only generally.

FIGS. A, B, and C indicate for purposes of exposition a set of Cartesiancoordinates related to the surface (a measurement surface) on which themeasuring wheel 11 runs. The X direction is considered to be in theprincipal direction of traverse of the indicator, that is, for example,along the length of the guide way 17. TheX axis would approximate atangent to the measuring wheel 11. The Z axis is defined in a directionnormal to the principal direction of traverse of the wheel andapproximately parallel to the axis of rotation to the wheel. The Y axisis, of course, normal to the X and Z axes and the XY plane isapproximately the plane in which the measuring wheel 11 lies.

As pointed out hereinafter, to obtain accurate readings the measuringdevice must also be properly angularly positioned, and therefore anangle a is defined as rotation about the Y axis as seen in FIG. C. Asused herein, this may also be referred to as skew of the measuringwheel. Another angle of interest is illustrated in FIG. A as 7 which isa measure of the rotation of the measuring instrument around the X axis,which may herein be referred to as tilt." It will be recognized that inthe drawings of this application the angles a and 'y are greatlyexaggerated for purposes of illustration. It should also be noted thatin the course of adjusting the angles of a measuring instrument, thecenters of actual rotation may not be and often are not at the center ofthe coordinate system just defined. Their exact position is typicallydetermined by the particular support 16 chosen.

As set forth in detail in U.S. Pat. No. 3,561,121, the measuring wheelpreferably has a relatively larger radius, and hence circumference,nearer one face and a relatively smaller radius nearer the other facewith the peripheral surface therebetween being arcuate. In theinstrument of FIGS. A, B, and C the line of maximum circumferentialextent around the wheel lies in a plane perpendicular to the wheel axisof rotation and is nearer the lower face of the wheel than the upperface. When the measuring instrument is aligned so that the axis ofrotation of the wheel is perpendicular to the Y axis (angle 7 equalszero), the line of maximum circumferential extent around the wheel liesin contact with the fixed portion 17 of the machine tool. Further, asthe measuring instrument is tilted up to increase the angle 'y (FIG. A),contact between the wheel 11 and fixed portion 17 lies along a line oflesser circumferential extent.

The support base 16 is a conventional element such as described in theaforementioned U.S. patents. For purposes of practice of this inventionvarious adjustments can be made by using the base in a conventionalmanner. The position of the instrument along the Y axis can be adjusted,typically, for example, by a dovetail (not shown). In addition toadjustment in the Y direction, means are provided for applying a springforce on the instrument for applying a relatively large (40 pounds beingrecommended for a TRAV-A-DIAL device) and preferably substantiallyconstant load between the measuring wheel and the measurement surface.This force assures frictional engagement and minimizes the possibilityof slippage of the wheel which could introduce substantial and randommeasurement errors. The metal elastic crowding (see U.S. Pat. No.3,307,265) that must be compensated for to assure accuracy arises fromthe force exerted by the measurement wheel on a small area of themeasurement surface. Even with relatively small forces the metal elasticcrowding may be substantial since the area of contact is concomitantlysmall. The effect increases with increasing force, but not linearly andsubstantial force is preferred, to minimize magnitude of changes ineffect in response to minor changes of force.

The base 16 also provides set screws or the like (not shown) for skewingthe instrument about the Y axis by an angle 7 and for tilting theinstrument about the X axis by an angle Movement of the instrumentduring adjustment for skew actually occurs about an axis displaced fromthe intersection of the measuring wheel with the measurement surface.Thus, as the angle a is changed slight rotation of the wheel inengagement with the measuring surface may also occur. Similarly thecenter of rotation of the instrument as the tilt is changed is displacedfrom the intersection between the wheel and measurement surface, and ithas therefore been desirable to merely remove or relax the springloading of the instrument against the measurement surface duringaccuracy adjustments to prevent overloading of the spring system andpossible damage to the instrument.

As pointed out in U.S. Pat. No. 3,561,121 it has been found that alocalized deformation phenomenon (termed metal elastic crowding") in thewheel and the measurement surface againstwhich it rides may cause anerroneously low distance measurement to be obtained with a frictionwheel measuring device. By tilting the measurement instrument about anangle 7 the effective circumference of the wheel in contact with themeasurement surface is decreased, thereby compensating for errorsattributable to metal elastic crowding. Thus, the accuracy of themeasuring instrument is assured and the distances indicated thereby areaccurately the distances actually traversed.

The effects of metal elastic crowding are reversible and compensationassures accuracy. Metal elastic crowding does not cause repeatabilityerrors.

Repeatability errors associated with operation of friction wheelmeasuring devices are produced by nonreciprocal deflection of both thecomponents of the machine tool itself and of the structure mounting themeasuring device on the machine tool. Such deflections are very slightbut they are sufficient in magnitude to produce repeatability errors ina friction wheel measuring device operated over large distances,particularly when there are repeated cycles of measurement. The factthat such a measuring instrument can detect the slight elastichysteresis of the machine tool testifies to the inherent great accuracyand sensitivity of such devices.

U.S. Pat. No. 3,56l,l provides a technique for mounting a friction wheelmeasuring instrument to compensate for, and thereby effectivelyeliminate repeatability errors due to nonreciprocal deflections.Repeatability errors are avoided without affecting accuracy.

The elements of a machine tool are typically constrained to have onlyone degree of freedom of movement. Machines are designed to minimizedeflections in all directions except the one intended motion.Unavoidable deflections in other directions lead to errors. Errors mayalso be introduced in worn machines or due to built-in tolerances givinglooseness" or backlash in directions other than the one intendeddirection. The measuring device is mounted on one element of the machineso that the measuring wheel is in frictional rolling engagement with thesurface on the second element, which is parallel to the direction ofdesired gross relative movement between the elements. In order tocompensate for repeatability errors, the magnitude of such errorgenerated when the device is mounted with the plane of rotation of thewheel parallel to the line of the gross relative movement is firstdetermined. The measuring device is then adjusted so that the plane ofmeasuring wheel rotation is displaced from parellism to the line ofgross relative movement by an amount which compensates for therepeatability error.

That is, the measuring device is intentionally mounted so that in an atrest state, the measuring wheel appears to track slightly skewed to thedirection of relative travel permitted between the two machine toolelements, and such angle of skew is by an amount that inherentlycompensates for repeatability errors which would otherwise beencountered. Such intentional initial skew mounting of the device,contrary to what would be expected, does not produce measurementaccuracy errors regardless of the amount of gross relative movementencountered between the elements. The terms gross relative movement" isused to designate the principal intended mode of movement relied upon tooperate the measuring device and to distinguish such move of movementfrom the undesired very small movements which produce repeatabilityerrors.

The technique of skewing the measuring instrument can be seen in FIG. C.The instrument is initially aligned with the plane of the wheel 11parallel to the XY plane. When a repeatability error is ascertained, theinstrument may be skewed by an angle a so as to no longer be parallel tothe X axis. The magnitude of the angle a to which the instrument shouldbe skewed is determined empirically.

Adjustment of the measuring instrument for repeatability error is setforth in detail in the aforementioned U.S. Pat. No. 3,561,120. Accordingto this technique the indicators 12 and 13 of the measurement instrumentare adjusted to show a zero reading with the instrument in a knownposition, preferably against a fixed stop defined by the structure ofthe lathe. The lathe carriage or the like is then moved several times(ten times, for example) back and forth as far as possible along thelathe bed from its original position. Because of the nonreciprocaldeflections of the lathe, the measuring device may show a measurementerror after the first round trip traverse of the carriage along thelathe bed, and such errors for each traverse accumulate additively sothat after several traverses the indicators will show a definite valuedifferent from the initial zero reading. The mounting base 16 is thenadjusted, with the carriage stationary, so that the fine measuringindicator 13 shows travel of the measuring wheel in a directionreturning the indicator towards a zero reading. The amount of returntowards zero depends on the length of traverse but is typicallysufficient to drive the dial indicator half way to a zero reading.

The carriage is then traversed again several times back and forth alongthe lathe bed and any accumulated residual error shown by the indicatoris noted. Skew of the measuring instrument is again adjusted togradually approach a skew mounting angle introducing a corrective errorequal in magnitude and opposite in sign to the repeatability errorintroduced by the nonreciprocal deflections of the machine tool. Suchadjustment can be tedious and time consuming and has required ratherprecise initial adjustment of the measuring device with the plane of themeasuring wheel parallel to the XY plane. It is desirable to avoid suchprecise initial adjustment and to ease the difficulty and reduce thetime to make the required adjustment for repeatability error withoutaffecting accuracy.

BRIEF SUMMARY OF THE INVENTION There is, therefore, provided in practiceof this invention means for reducing the sensitivity of a friction wheeldistance measuring instrument to skew tracking by providing a pluralityof randomly spaced-apart ridges on the periphery of the wheel, and byhaving the long axis of the ridges transverse to the circumference ofthe measuring wheel.

DRAWINGS These and other features and advantages of the presentinvention will be appreciated as the same becomes better understood byreference to the following detailed description of a presently preferredembodiment when considered in connection with the accompanying drawingswherein:

FIGS. A to C illustrate a prior art measuring instru ment;

FIG. 1 illustrates in side view a measuring wheel for practice of thisinvention;

FIG. 2 illustratesin end view a fragment of the periphery of the wheelof FIG. 1; and

FIG. 3 illustrates schematically ridges on the wheel of FIG. 1.

DESCRIPTION The ultimate cause of repeatability error in a frictionwheel distance measuring instrument is nonreciprocal deflection of themachine tool and the bracketry by which the measuring instrument ismounted thereto. These deflections produce (1) skew tracking of themeasuring wheel relative to the direction of gross relative movementbetween the measuring wheel and the measurement surface; (2) variationsin the tilt ('y) of the metering wheel relative to the measurementsurface; or (3) variations in the force of engagement of the measuringwheel with the measurement surface. These three effects may be producedsimultaneously or separately by nonreciprocal deflection of the machinetool and the mounting structure for the measuring instrument.

The deflections of the machine tool and mounting structure arenonreciprocal in that they are different in nature and magnitude for onedirection of travel along the machine tool than for travel in theopposite direction. If the elastic deformations were reciprocal, thatis, the same in both directions, the measuring instrument would producerepeatable results. The degree of nonreciprocity and hence therepeatability error is not readily predictable and is thereforedetermined empirically by simply measuring the repeatability error.

The measuring instrument has six degrees of freedom relative to themeasurement surface, namely, translation along each of the threedirections and rotation or pivotaing about each of the three axes. Onlythree of these 6 of freedom are actually of concern as regardsmeasurement accuracy or repeatability. Rotation of the measuringinstrument about the Y axis (angle a) produces skew tracking. Rotationof the instrument about the X axis (angle 7) produces a variation in theeffective diameter of the metering wheel 11. Translation of themeasuring instrument along the Y axis may produ ce a variation of theforce of engagement of the measuring wheel with the measurement surface,thereby producing a variation in the magnitude of the metal elasticcrowding.

The deflection-induced tendency of the measuring wheel to track skew tothe direction of gross relative movement of the instrument along the Xdirection is not directly a source of concern. Skew tracking of themeasuring wheel, however, produces a change in the tilt 'y of themeasuring wheel and often, also, a difference between the force ofengagement of the wheel with the measurement surface during movement ofthe instrument in opposite directions. X axis translation turns out tobe no problem at all. Z axis translation is either no problem due tosliding of the wheel parallel to its axis of rotation, or because of theforceful engagement of the metering wheel with the measurement surfaceit is reflected as X axis rotation (angle 7) by reason of deflection ofthe mounting structure for the measuring instrument. Z axis rotation isreflected as X axis translation.

In summary, because of the various cross coupling effects between thedifferent degrees of freedom, skew tracking of the measuring wheel mayproduce changes in the'tilt angle of the measuring wheel relative to themeasurement surface and hence of effective wheel diameter. Crosscoupling effects may also cause skew tracking to be manifested aschanges in the force of engagement of the metering wheel with themeasurement surface, thereby changing the effect of metal elasticcrowding. Both of these can give repeatability errors.

FIG. 1 illustrates in side view a typical measuring wheel 11 forpractice of this invention. The wheel 11 illustrated in FIG. 1 isexaggerated in proportion for purposes of exposition. As illustrated inthis presently preferred embodiment, the wheel has a first face 21 andan opposing face 22 parallel thereto on the opposite side of the wheel.The periphery 23 of the wheel has an arcuate profile with a radius ofcurvature which is preferably greater than the radius of the wheel andan assymetrically positioned locus of centers for the curvature so thatthe line of maximum circumferential extent 24 is much nearer the lowerface 21 than the upper face 22 of the wheel. Thus, as pointed outpreviously, the circumferential extent of the wheel near the upper face22 is less than the circumferential extent of the wheel adjacent thelower face 21. Therefore, by tilting the wheel the effectivecircumference can be varied to obtain any desired degree of accuracy ina manner described in greater detail in US. Pat. No. 3,561,121. Theradius of the arcuate surface may be less than or equal to the radius ofthe wheel.

Previously, it has been the practice to grind the measuring wheel for ameasuring instrument in a direction such that the grinding marks extendin the circumferential direction around the periphery of the wheel. Thisis done for greatest ease of manufacture and ability to maintaincircularity of the wheel and obtain a precisely predeterminedcircumference. Measuring wheels are sometimes polished after grinding.

In practice of this invention, on the other hand, it is preferred togrind the measuring wheel to its final dimensions in a directiontransverse to the circumference of the measuring wheel. This results ina plurality of randomly spaced apart sharp ridges 26 on the periphery ofthe wheel with the long axis of the ridges transverse to thecircumference. Significantly improved results are obtained thereby andrepeatability adjustment greatly simplified. Randomness of the spacingof ridges is of significant importance as pointed out in detailhereinafter. I

As pointed out hereinafter it is important that the maximum averagespacing between ridges be about 10 mils (i.e., 0.010 inch) and that theminimum average spacing between the ridges be greater than about 1 microinch. Preferably the average spacing between the ridges is about 1 milfor optimum results. It is important that the angle between the slope ofthe ridge adjacent the crest thereof and a tangent to the wheel at thecrest of the ridge be substantially greater than the angle of staticfriction between the'wheel and a surface to be measured so that slippageis precluded. It is also important that the ridges have an averagelength transverse to the circumference of the wheel more than aboutthree. times the average spacing between the ridges. This assures that amaximum force is exerted by the ridges in the circumferential directionof the wheel without substantial increase in the forces exerted in theaxial direction. It also assures that the wheel will follow the sametrace in a measurement surface despite slight changes in tilt of thewheel or translation in the Z direction.

For purposes of exposition it will be considered that the distancemeasuring instrument is mounted on a lathe carriage for traversal in theprincipal direction of the lathe bed, that is, the X direction. Themeasuring wheel 11 of the distance measuring instrument engages aguideway of the lathe bed for measuring the extent of traversal of thecarriage along the bed. The lathe guideway is thus the measurementsurface. When a prior art smooth or circumferentially ground frictionmeasuring wheel is rolled along the guideway, a repeatability error mayoccur since a selected point on the measuring wheel may contact a givenpoint on the guideway during the forward portion of the traverse and beslightly displaced from that given point when the traversal is reversed.This is the case since there is nothing to assure that the measuringwheel follows exactly the smae trace during the forward and reverseportions of the cycle. Exact repeatability could be obtained if themeasurement surface were in the form of a rack having teeth and themeasuring wheel were in the form of a pinion having teeth engaging thoseon the rack. Because the pinion must necessarily follow the rack onreturn, absolute repeatability is assured. Some prior art measurementwheels have included a pinion that embosses teeth on a measurementsurface and necessarily returns along said teeth for repeatability.

A regular rack and pinion arrangement, whether formed initially on thesurface or formed by embossing, destroys the ability to adjust thefriction wheel measuring instrument for accuracy by tilting the wheel.So long as the wheel is constrained to follow a trace of periodic teethit must return to the same point from which it started, even if thecircumference of the wheel is changed slightly as is done by tilting thewheel having an arcuate peripheral surface. if one grinds the peripheryof an ordinary pinion so that the teeth are only half as high, it willstill roll along a rack in the original manner. The gear ratio is notchanged. With regularly spaced teeth repeatability is obtained at thesacrifice of ability to adjust to obtain a desired degree of accuracy.

To obtain accuracy it is important that the measuring wheel return tothe origin of its travel along exactly the same trace that it made inleaving the origin. For repeatability it should follow this tracedespite minor changes in tilt of the wheel or changes in the engagementforce between the wheel and measurement surface. One needs, however, tobe able to provide a completely new trace, when desired, in conjunctionwith deliberate tilting of the wheel to adjust accuracy so that apre-existing trace has no influence.

There are, therefore, provided sharp ridges on the wheel peripheryhaving their long axis running transverse to the wheel circumference.The ridges are randomly spaced apart rather than periodic, that is, thespacing between the ridges varies from ridge to ridge in a randommanner.

To utilize the measuring instrument it is mounted on the machine tool ina conventional manner, accuracy is measured and then adjusted, asrequired, by tilting the instrument to change the effective measuringwheel circumference. If that were all that was done, the tilted wheelwould have ridges adjacent the same trace previously made. This wouldbehave like an ordinary rack and no improvement in accuracy wouldresult. Therefore, prior to tilting the instrument it is withdrawn fromengagement with the measurement surface so that the ridges on the wheelno longer engage the surface. The measuring wheel is rotated prior tore-engaging the wheel with the measurement surface. The angle of tilt ofthe instrument may be adjusted either before or after rotating thewheel.

By withdrawing the wheel from engagement with the measurement surfaceand rotating the wheel, the randomly spaced ridges thereon cannot matewith a trace previously made in the measurement surface. In the absenceof such mating, the trace followed by the measuring wheel on making anew traversal will effectively obliterate any prior trace and form a newone. When the cycle is reversed, the ridges on the measuring wheel willfollow the new trace rather than the old one. Thus, the importance ofrandom spacing of the ridges becomes apparent. If the ridges wereperiodically spaced, rotation of the wheel while disengaged from themeasuring surface would have no effect since the wheel would necessarilyengage the old periodic trace when brought back into engagement with thesurface. The teeth on a pinion engage the teeth on a conventional rackby falling into place irrespective of rotational position of the pinion.

The improved technique for setting the accuracy of the measuringinstrument has only a few simple steps. Typically, a 6-inch longmeasuring standard is employed which coincides with the circumference ofthe measuring wheel on the commercial English system TRAV-A-DlALinstrument, for example. A conventional dial test indicator ispositioned on one of the measuring flats of the 6-inch standard andzeroed to 0.0001 inch; this is done after thehousing has been alignedparallel to the X axis by known practices. The measuring instrument isalso zeroed. The lathe or other machine tool is then traversed exactlysix inches so that the dial test indicator again zeroes on the secondmeasuring flat of a six-inch standard. Any deviation of the indicationon the measuring instrument from zero i.e., 6 inches) is a deviationfrom accuracy. If the measuring instrument reads more than 6 inches'itmeans that the instrument should be adjusted to reduce the tilt angle.If, on the other hand, the measuring instrument does not quite read sixinches it is necessary to increase the tilt-up angle even further untilmeasuring accuracy is achieved.

Once the magnitude and direction of the measurement error have beennoted, the spring force pressing the measuring wheel against themeasurement surface is removed. This, however, leaves the wheel still incontact with the measurement surface in the typical machine shop set up.The measuring instrument is therefore retracted so that the wheel isdisengaged from the measurement surface. The measuring instrument istilted as required to enhance accuracy. The index wheel is also rotatedby an amount that assures that the same ridges as before are not inengagement with the same spot on the measurement surface. A rather smalldegree of wheel rotation is sufficient since the ridges have closespacing. The degree of shifting needs to be different than an integralnumber of full rotations of the wheel, that is, different from zerorotation, or one full revolution. It will be apparent, of course, thateffective rotation is all that is necessary and that, if desired,traversal of the retracted measuring instrument relative to themeasurement surface actually accomplishes the same effect of moving thewheel from its earlier trace. Although this is effective, it istypically easier in practice to merely re-index the wheel. It will alsobe recognized that the key step is displacing the perimeter of the wheelrelative to the measurement surface and that this could be accomplishedby sliding the wheel on the surface rather than by disengaging andindexing the wheel. This, however, burnishes the crests off the ridgeseven on the hardened wheels and the benefits of the ridges in enhancingrepeatability are gradually lost. Any manner of displacement between thewheel and measurement surface can suffice to give effective rotation ofthe wheel. Retraction and rotation is greatly preferred.

After the wheel has been turned relative to the measurement surface, itis re-engaged with the measurement surface and the spring force isreapplied. The accuracy is then measured in exactly the same manner and,if necessary, the steps are repeated until a desired degree of accuracyis obtained.

Retraction of the measuring instrument from the measurement surface canbe accomplished by manipulating the conventional mounting base by whichit is attached to the machine. However, it is found that once the springforce has been removed it is a simple matter to manually force theinstrument away from the measurement surface since the resilience of themounting and the machine tool typically permits sufficient deflectionthat the wheel can be freely indexed without engagement with themeasurement surface. In practical operation, the key steps aredisengagement of the wheel from the measurement surface prior toeffective rotation of the wheel followed, by re-engagement for purposesof measurement. The tilt of the measuring instrument for actuallyadjusting accuracy can be accomplished either before or after indexingthe wheel and can be with the wheel in contact with the measurementsurface, although it is preferred that it be disengaged. The springforce should be relieved before tilting to avoid overloading.

Referring again to FIGS. A through C, pads 28 are formed on theinstrument case 10, and these pads are carefully ground so as to beparallel to the plane of the measuring wheel 11. In adjusting the skewof the measuring instrument, a dial indicator or the like is mounted onthe lathe bed and indexed against one of the pads. The carriage is thentraversed so that the dial indicator is indexed against the other pad28. The skew of the measuring instrument is then adjusted to make thewheel parallel to the X axis. Previously, the deviation for parallelismof the wheel from the X axis was adjusted to less than about 0.0005 inchas measured between the two pads which are about 1 13/32 inches apart.From this starting point the skew of the measuring instrument wasadjusted to eliminate any repeatability error. Typically, three or fouradditional adjustments of the skew angle were necessary to completelyeliminate the repeatability error.

In practice of this invention wherein randomly spaced apart ridgesextend transverse to the wheel circumference, a similar adjustment isused for repeatability error; however, the initial skew adjustment maybe made to only about 0.004 inch as measured between the pads 28 and itis quite common that if any additional skew adjustment is required, onesuch adjustment suffices. This ability of the improved measuringinstrument to compensate for repeatability error with a skew angle abouteight times as great as before is believed to lie in the formation of atrace by the measuring wheel which is followed by the wheel uponreversal.

It is desirable to have a high coefficient of friction between the wheeland the measurement surface in the X direction (circumferentialdirection) as the wheel is used in order to minimize any slippage thatmight occur and result in measurement error. The ridges which engage themeasurement surface provide such a high coefficient of friction. At thesame time it is desirable to have low coefficient of friction in the Zdirection (transverse direction) so as to minimize the degree of crosscoupling which tends to change the tilt of the wheel and the engagementforce, both of which contribute to repeatability error. Thus, a highcoefficient of friction in the X or peripheral direction enhancesrepeatability and a low coefficient of friction in the Z or transversedirection enhances repeatability.

The randomly spaced ridges which have their long axes extending in thetransverse direction provide a high coefficient of friction in the Xdirection without substantially increasing the coefficient of frictionin the Z direction. The same would not be true of pits or spikes formeduniformally over the wheel perimeter such as might be obtained bysandblasting the periphery or by making the wheel porous. Thus, it isfound that in order to provide a high ratio between the coefficient offriction in the X direction and the coefficient of friction in the Zdirection, the average length of the ridges should be more than aboutthree times the average spacing therebetween. Average length and spacingare used herein in a sense that can be numerical average, mean, ormedium without substantial difference. An average value is necessarysince the actual spacing between the ridges is random. Typically it isalso found when the ridges are produced by grinding that the length isalso random. Preferably the average length of the ridges is five to tentimes the average spacing since this assures a high ratio ofcoefficients of friction and this is readily obtainable without unduemanufacturing problems.

Another reason for having ridges of a substantial average lengthrelative to the average spacing therebetween is to assure that the wheelactually remains in the same trace despite any minor changes in theangle of tilt or translation in the Z direction during a traverse. Ifthe ridges were too short, a small tilt of the wheel could cause it tomiss the original trace and a repeatability error would again beobserved.

It is desirable that the ridges be small and sharp in order to form agood trace and to keep them from leaving the trace during reversetravel. The maximum average space between V-shaped ridges on a wheelperiphery is determined by the formula 40 mils is obtained. This issubstantially an absolute I maximum; as a practical matter, a maximum ofabout 10 mils average groove spacing is actually preferred. In apreferred embodiment the wheel is ground so that the average spacingbetween the ridges is about 1 mil (10 inch) which assures high accuracyand excellent repeatability without substantial manufacturingdifficulties. It is preferred that the average spacing between ridges begreater than about 1 micro inch since the wheel is then so smooth thatno substantial increase in the ratio of coefficients of friction can beobtained.

It is also important that the ridges be sharp so as to properly engagethe measurement surface. When the ridges are sharp the pressure (forceper unit area) of the wheel against the measurement surface is quitehigh and a trace is formed in the measurement surface. Conventionalgrinding of the measurement wheel in a direction transverse to itscircumference produces very sharp ridges. The crests that are formed aretypically in the same order of size as the crystal size of the metalmaking the wheel. With a hardened steel wheel, which is preferred inpractice of this invention, extremely small grain size is typical andgrinding produces very sharp, randomly spaced ridges. Thus by statingthat the ridges are sharp, it is meant that the degree of sharpness issubstantially that produced by grinding with a conventional abrasivegrinding wheel. Such sharp edges may become slightly dulled during useof the measuring instrument, however, such normal wear does notsubstantially degrade performance of the instrument.

Any significant dulling of the ridges, such as might occur by polishing,burnishing, electrolytic etching or the like will degrade performance ofthe measuring instrument. It should be noted that such intentionaldulling of the sharp ridges would need to be substantial before theinstrument would cease to perform as hereinabove described. Thus, bysharp, in defining the ridges, is meant that the peaks are substantiallyas produced by conventional grinding with no intentional radius or otherintentional dulling at the crests of the ridges.

Not only must the crest of the ridge be very narrow but also the slopeof the sides of the ridge must be steep. Thus, the angle between theslope of the ridges adjacent the crest of the ridge and a tangent to thewheel at the crest must be substantially greater than the angle ofstatic friction between the wheel and the surface to be measured (thecoefficient of friction between two surfaces is the tangent of the angleof static friction). FIG. 3 illustrates the crest of a pair of adjacentridges 26 spaced apart by distance S. The slope 6 between the side ofthe ridge and a tangent to the wheel should be substantially greaterthan the angle of static friction. If it is not substantially greaterthan the angle of friction, the ridge may slide out of the tracepreviously made. The wheel is preferably hard steel, harder than themeasurement surface so that the ridges do not become burnished and dullafter prolonged use.

Although described in greatest detail hereinabove with respect to ameasuring wheel deliberately crowned for adjustment of measurementaccuracy, the principles of this invention are also applicable to ameasuring instrument in which the periphery of the measuring wheel is inthe form of a right circular cylinder; Thus, such a cylindrical wheelwith randomly spaced sharp ridges extending transverse to thecircumference of the wheel prevents repeatability error independent ofthe tilting technique for compensating measurement errors.

In one embodiment of measuring instrument, a wheel having a rightcircular cylinder for a periphery is used. If it is found that the rightcircular cylinder is not quite of the desired circumference, or if thereis some other measurement error, an external compensation for thismeasurement error can be used. Typically, such an instrument has anelectronic read-out wherein rotation of the measuring wheel is senseddirectly or indirectly by an electronic sensor, such as, for example, aphotocell. With suitable conventional circuitry this rotation of themeasuring wheel can be translated into a distance or angular measurementin any selected units without substantial difficulty. Several techniquesare then available for adjusting the measurement indicated by theinstrument to coincide with the true distance traversed by the wheel.These techniques spread the measured deviation from accuracy over theentire measurement distance so that high accuracy can be obtained overshort or long distance measurements, or over measurements of angle. Withsuch an adjustable electronic read-out,

a wheel having a right circular cylindrical periphery can be used inlieu of the arcuate periphery hereinabove described and illustrated.

Even when the accuracy of measurement of the instrument iselectronically adjusted repeatability errors may be introduced due themechanical hysteresis of the machine tool on which the instrument ismounted. The randomly spaced sharp ridges still introduce a trace in themeasurement surface and thereby prevent reproducibility errors. The factthat the ridges are randomly spaced apart becomes less important whenthe wheel has a right circular cylindrical periphery than when it has anarcuate periphery since measurement errors are compensatedelectronically and the adjustment technique wherein the instrument isdisengaged from the surface and tilted is not required. Sharpness andsubstantial length transverse to the wheel circumference are still ofsignificant importance. Randomness remains of some importance, however,since the preferred technique for producing the sharp ridges is byconventional grinding. This is by far the most economical technique forpreparing sharp ridges in the periphery of the wheel. The measuringwheel is preferably hardened steel and techniques for cutting regularlyspaced ridges on the periphery thereof are quite expensive. If the wheelis provided with sharp transverse ridges prior to hardening, thehardening operation may introduce distortion, which would produce a'short order measurement error, which cannot readily be compensated byelectronic techniques.

Although limited embodiments of this invention have been described andillustrated herein, many modifications and variations will be apparentto one skilled in the art. Thus, for example, although the ridges arestated to be random, and such is apparently the case when they areproduced by grinding, it will also be apparent to one skilled in theart, that pseudo-random ridges having a predetermined nonuniformaperiodic spacing can also be employed. Long ridges that run clearacross the periphery from face-to-face of the wheel would be preferredbut as a practical matter in a typical grinding operation the ridges areshorter and may be three to ten times the average spacing of the ridges.Many other modifications and variations will be apparent to one skilledin the art and it is therefore to be understood that within the scope ofthe appended claims the invention may be practiced otherwise than asspecifically described.

I claim:

1. In a measuring instrument which includes a wheel for engaging androlling along a surface to be measured, the surface being defined by oneof two elements which are movable relative to each other along a pathessentially parallel to the surface, the periphery of the wheel beingarcuately curved convex away from the wheel axis of rotation whereby theeffective radius of the wheel from said axis to the point of engagementwith said surface is adjustable by tilting said axis relative to saidsurface, means for measuring wheel rotaridges transverse to thecircumference of the wheel.

2. In a measuring instrument as defined in claim 1, the improvementwherein the ridges have an average length more than about three timesthe average spacing between the ridges.

3. In a measuring instrument as defined in claim 1, the improvementwherein the ridges have an average spacing therebetween less than about0.010 inch.

4. In a measuring instrument as defined in claim 3, the improvementwherein the ridges have an average spacing therebetween greater thanabout 10 inch.

5. In a measuring instrument as defined in claim 3, the improvementwherein the ridges have an average spacing therebetween in the order ofabout 10 inch.

6. In a measuring instrument as defined in claim 1, the improvementwherein the ridges comprise grinding marks from a grinding tool movingin a direction substantially parallel to the axis of rotation of thewheel.

7. In a measuring instrument as defined in claim 1, the improvementwherein the crests of the ridges are substantially as sharp as producedby grinding.

8. In a measuring instrument including a wheel for engaging and rollingalong a surface to be measured, the surface being defined by one of twoelements which are movable relative to each other along a pathessentially parallel to the surface, means for mounting the instrumentto the other of the two elements with the wheel adjacent the surface andwith the wheel plane of rotation fixed substantially parallel to saidpath, and means for measuring wheel rotation for conversion to a desiredmeasurement, the improvement comprising:

a plurality of randomly spaced apart ridges on the periphery of thewheel having the long axis of the ridges transverse to thecircumferential extent of the wheel, the ridges having an averagespacing therebetween less than about 0.010 inch.

9. In a measuring instrument as defined in claim 8 wherein the ridgeshave an average spacing therebetween greater than about 10" inch.

10. Ina measuring instrument as defined in claim 8 wherein the ridgeshave an average spacing therebetween in the order of about 10* inch.

' g;;g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No-3,771,Q 2 8 Dated ngx h z 13 1223 Inventor(s) Irven j ulver It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Col. 4, 1111.233, reads Y i should read --a, C01. 7, line 19 reads"pivotaing", should read --pivoting-- Col. 8, line 68, reads "smae",should read" --same-- Signed and sealed this ll th day of May l97 L V(SEAL) Attest:

EDwARD 1'-'I.FLETCHER,J}E. c M RSHALL DANN Attesting Officer v vCommissioner of Patents

1. In a measuring instrument which includes a wheel for engaging androlling along a surface to be measured, the surface being defined by oneof two elements which are movable relative to each other along a pathessentially parallel to the surface, the periphery of the wheel beingarcuately curved convex away from the wheel axis of rotation whereby theeffective radius of the wheel from said axis to the point of engagementwith said surface is adjustable by tilting said axis relative to saidsurface, means for measuring wheel rotation for conversion to a desiredmeasurement, and means for mounting the instrument to the other of thetwo elements with the wheel adjacent the surface and with the wheelplane of rotation fixed substantially parallel to said path, theimprovement comprising: a plurality of randomly spaced apart ridges onthe periphery of the wheel having the long axis of the ridges transverseto the circumference of the wheel.
 2. In a measuring instrument asdefined in claim 1, the improvement wherein the ridges have an averagelength more than about three times the average spacing between theridges.
 3. In a measuring instrument as defined in claim 1, theimprovement wherein the ridges have an average spacing therebetween lessthan about 0.010 inch.
 4. In a measuring instrument as defined in claim3, the improvement wherein the ridges have an average spacingtherebetween greater than about 10 6 inch.
 5. In a measuring instrumentas defined in claim 3, the improvement wherein the ridges have anaverage spacing therebetween in the order of about 10 3 inch.
 6. In ameasuring instrument as defined in claim 1, the improvement wherein theridges comprise grinding marks from a grinding tool moving in adirection substantially parallel to the axis of rotation of the wheel.7. In a measuring instrument as defined in claim 1, the improvementwherein the crests of the ridges are substantially as sharp as producedby grinding.
 8. In a measuring instrument including a wheel for engagingand rolling along a surface to be measured, the surface being defined byone of two elements which are movable relative to each other along apath essentially parallel to the surface, means for mounting theinstrument to the other of the two elements with the wheel adjacent thesurface and with the wheel plane of rotation fixed substantiallyparallel to said path, and means for measuring wheel rotation forconversion to a desired measurement, the improvement comprising: aplurality of randomly spaced apart ridges on the periphery of the wheelhaving the long axis of the ridges transverse to the circumferentialextent of the wheel, the ridges having an average spacing therebetweenless than about 0.010 inch.
 9. In a measuring instrument as defined inclaim 8 wherein the ridges have an average spacing therebetween greaterthan about 10 6 inch.
 10. In a measuring instrument as defined in claim8 wherein the ridges have an average spacing therebetween in the orderof about 10 3 inch.
 11. An instrument for measuring distance traversedalong a measurement surface comprising: a measuring wheel having aperiphery for frictionally engaging a measurement surface, the peripheryhaving an arcuate profile, the surface being defined by one of twoelements which are movable relative to each other along a pathessentially parallel to the surface, means for mounting the instrumentto the other of the two elements with the wheel adjacent the surface andwith the wheel plane of rotation fixed substantially parallel to saidpath; means for indicating the distance traversed by the wheel along themeasurement surface; and means on the periphery of the wheel for forminga trace on the measurement surface with a relatively higher coefficientof friction in the direction of traversal and a relatively lowercoefficient of friction in a direction transverse thereto, and forrepetitively following the formed trace, and for ignoring any priortrace not coincident with the formed trace, said trace forming meanscomprising a plurality of randomly spaced apart ridges on the peripherywith the long axis of the ridges extending transverse to thecircumference of the wheel.
 12. In a measuring instrument as defined inclaim 1 the improvement wherein the ridges extend across the peripheryfrom face to face of the wheel.